1. Field of the Invention
Embodiments of the invention generally relate to a method for depositing silicon and nitride layers using a plasma enhanced chemical vapor deposition (PECVD) process.
2. Background
Substrate size expansion has been the enabler of the TFT-LCD industry. Since TFT-LCD production using substrates having a plan area of about 2000 cm2 started in 1993, the growth rate of substrate size has been almost exponential, enlarging more than 30 times in plan area in 13 years. This rapid growth of substrate size has been very challenging for display manufactures, material suppliers and equipment makers; and a driving force for everyone to improve. Many challenges were faced in scaling up the plasma enhanced chemical vapor deposition (PECVD) reactors and PECVD processes to accommodate substrates having a 2160×2460 mm2 plan area. The most severe challenges were maintaining the integrity and stability of larger electrodes, maintaining substrate temperature uniformity, maintaining gas distribution uniformity, and last but not least, maintaining the same or better film quality achieved during processing 2000 cm2 substrates without sacrificing productivity.
As the substrate size has grown, thermal contraction of glass substrate has become more problematic for the photo engraving exposure process. Among TFT-LCD production processes, the most commonly used and highest process temperature is 350 degrees Celsius for the PECVD silicon nitride (SiNx) gate dielectric layers and amorphous silicon (a-Si) active layers. This relatively high temperature and other associated process conditions were arrived at for the first single-chamber PECVD for TFTs in and around the LCD industry's 2000 cm2 substrate timeframe. Reducing the process temperature even by 60 degrees Celsius can drastically reduce thermal contraction. An additional or alternative benefit of lower temperature processing is the possibility to use a less expensive glass substrate which may have a higher coefficient of thermal expansion. A likely further benefit of reducing process temperature is improved PECVD hardware reliability and system utilization. While the benefits of lower temperature are several, current PECVD processes are optimized to obtain required quality with high deposition rate at 350 degrees Celsius; therefore simply lowering process temperature without discovering solutions for process conditions would degrade film quality. Additionally, reducing deposition rate to improve film quality at lower temperatures is not a viable solution since sacrificing deposition rate would reduce throughput, thereby making such a process impractical for a production systems.